This manuscript discussed chemical composition and source apportionment of PM2.5 in three northern Chinese cities based on high-time-resolution monitoring equipment. Furthermore, the authors qualitatively explored the mechanisms of secondary aerosols formation. And the authors also compared the changes in the chemical composition and sources of PM2.5 in the three pilot cities over past decades. At the end of the article, the authors provided policy recommendations for sustained air quality improvement in the future. In general, this article provides valuable data for local governments to develop scientific pollution control strategies. I recommend this article for publication on Atmospheric Chemistry and Physics. However, some issues should be addressed before the publication.

Major comments:

1.    In Text S1 from supplement, the error fraction of all chemical species was used to 10% for calculation of uncertainty dataset inputted receptor model. Is it reasonable to choose the same error fraction for different instruments? 

2.    In Sect3.2, the secondary sources from three cities were resolved by HERM model. And one factor named secondary nitrate plus sulfate was identified in Xi’an and Shijiazhuang (Fig. S6 and S7), but two factors named secondary nitrate and secondary sulfate were identified in Beijing (Fig. S8). What causes such differences in sources identification among cities? Moreover, the secondary sources were characterized by high EV values for SO42- , NO3-, NH4+, and those three species are inorganic aerosols. Secondary organic aerosols (SOA) were not identified in this study, dose SOA was ignored in this study? 

3.    In Sect3.3, the authors explored the potential function of aqueous-phase reaction to secondary aerosol formation based on data during nighttime. It should be noted that aqueous-phase reaction may also promote secondary aerosol formation during daytime. I suggest that the authors take the data from daytime hours into account when discussing the role of the aqueous-phase reaction.  

Specific Comments:

1.    line18 & 352： change “progress of” to “process of”

2.    line 22-23: change "but is no longer important contributor...." to "but is no longer an important contributor...."

3.    line 27：change “According to source variations, to “According to sources variations,”

4.    line 49：change “In perspective of PM2.5 sources” to “In terms of PM2.5 sources”

5.    line 67：change “Liu et. al (2019)” to “Liu et al. (2019)”

6.    line 73：change “should be pointed out previous researches” to “should be pointed out that previous studies”

7.    line 78 & 85-86：change “are located to” to “are located in”

8.    line 237：change “With increasing of the PM2.5 mass concentration” to “With increases of the PM2.5 mass concentration”

9.    line 431&454：change “atmosphere oxidation capacity” to “atmospheric oxidation capacity”